We didn't get that exactly when I was pregnant cos we knew we were having a boy from 16 weeks due to an amnio & told everyone straight away. What we did have was stuff like:

"You can tell you are carrying a boy because you are carrying high".
"You can tell you are having a boy because you are so small".
"You can tell you are having a boy because you are all belly".

Honestly people just make it up as they go along.